SampleTable.cpp: Fixed a regression caused by a fix for bug

28076789.

Detail: Before the original fix
(Id207f369ab7b27787d83f5d8fc48dc53ed9fcdc9) for 28076789, the
code allowed a time-to-sample table size to be 0. The change
made in that fix disallowed such situation, which in fact should
be allowed. This current patch allows it again while maintaining
the security of the previous fix.

Bug: 28288202
Bug: 28076789
Change-Id: I1c9a60c7f0cfcbd3d908f24998dde15d5136a295